An electrode comprising synthetic high-pressure high-temperature diamond material, the diamond material comprising a substitutional boron concentration of between 1×1020 and 5×1021 atoms/cm3 and a nitrogen concentration of no more than 1019 atoms/cm3. The electrode has a ΔE3/4-1/4 as measured with respect to a saturated calomel reference electrode in an aqueous solution containing 0.1 M KNO3 and 1 mM of Ru(NH3)63+ selected any of less than 70 mV, less than 68 mV, less than 66 mV, and less than 64 mV, and/or a peak to peak separation ΔEp as measured with respect to a saturated calomel reference electrode in an aqueous solution containing 0.1 M KNO3 and 1 mM of Ru(NH3)63+ selected any of less than 70 mV, less than 68 mV, less than 66 mV, and less than 64 mV.